I'm new to rolling updates, but if one of the eventual targets of
Tumbleweed is servers, there needs to be way to roll a consistent set
of updates to first a test server, and then to production servers.

I wouldn't use Tumblweed for servers, but evergreen (but may be I'm wrong)

From my understanding, a lot of people run Arch on servers because

they prefer a slow continuous upgrade process instead of a big upgrade
every year or two.

I don't know if the long term goal of Tumbleweed is to support that
model or not.